Recently, the receptor proteine tyrosine phosphatase (RPTP[unreadable]?) has been identified as involved in bone formation. RPRP[unreadable]? was strongly induced between day 5 and 25 of primary osteoblast differentiation, more so than other genes known to be induced upon terminal osteoblast differentiation such as Bglap, Ibsp, Dmp1, Akp2 and PheX. RPTP[unreadable]? ?has previously been described mainly in neuronal cells, and was the first described receptor for the heparin-binding molecule pleiotrophin (PTN), a small chemokine of 15- 17 kDa, that was also independently identified as Osteoblast-stimulating factor-1 (OSF- 1). Mice engineered to over-express PTN show a marked increase in intramembraneous bone formation and multiple effects on long-term bone growth. The molecular mechanism with which PTN exerts its stimulatory effects is not known in detail. A current hypothesis is that PTN-induced dimerization of RPTP[unreadable]?? ?results in loss of intracellular RPTP[unreadable]?? phosphatase activity. In the absence of PTN signaling, RPTP[unreadable]?? ?is shown to dephosphorylate [unreadable]-catenin, resulting in the formation of a [unreadable]-catenin/a-catenin/E-cadherin molecular complex, which tethers actin filaments to the cell membrane. This is necessary for normal cell-cell adhesion. PTN signaling though the RPTP[unreadable]?? ?receptor leads to decay of [unreadable]catenin/E-cadherin complex formation, due to loss of tyrosine dephosphorylation of [unreadable]-catenin, disruption of the actin cytoskeleton and loss of cell-cell adhesion. Our aims are to perform in vitro studies of the entire system consisting of pleiotrophin, RPTP[unreadable]??? and RPTP[unreadable]?? phosphatase-domain substrates. These studies will be highly relevant for understanding the molecular interactions between the proteins in this signaling network. Also, more importantly, biophysical characterization and three dimensional structural understanding of this newly discovered signaling system will open up a new field and lead to future structure-based drug design of new PTN/RPTP[unreadable]??? small-molecule binding agents that might be able to treat osteoarthritis, osteopenia/osteoporosis, and other diseases associated with changes in bone density. PUBLIC HEALTH RELEVANCE: The receptor protein tyrosine phosphatase [unreadable]?? ?(RPTP[unreadable]? ) is a key transmembrane protein that regulates different aspects of normal cell function. We plan to study the interactions between this protein and its known ligand pleiotrophin plus other proteins know to interact with RPTP[unreadable]?, using a variety of biophysical techniques.